Fill level limit switches of this type are applied in many branches of industry, especially in the chemical and food industries. They serve for detecting a limit level and are e.g. used as overfill guards or protection against pumps running empty.
DE-A 44 19 617 describes a device for establishing and/or monitoring a predetermined fill level in a container. The device includes:                a mechanical oscillatory structure placed at the height of the predetermined fill level,                    which exhibits a membrane, or diaphragm, and two mutually separated oscillation bars formed thereon,                        an electromechanical transducer,                    which in operation excites the oscillatory structure to oscillate with oscillations, such that the oscillation bars execute oscillations perpendicular to their longitudinal axis, and                        a receiver- and evaluation-unit, which uses the oscillation to establish and/or monitor whether the predetermined fill level has been reached, or not.        
The oscillation bars have flat, mutually parallel paddles on their membrane-far ends. A normal to the paddle surfaces is perpendicular to the longitudinal axis of the paddles.
The electromechanical transducer has at least one transmitter, at which an electrical, transmitted signal is applied and which excites the mechanical oscillatory structure to oscillate. A receiver is provided, which picks up the mechanical oscillations of the oscillatory structure and transforms such into an electrical, received signal. The evaluation unit obtains the received signal and compares its frequency with a reference frequency. It produces an output signal, which indicates that the mechanical oscillatory structure is covered by a fill material, if the frequency has a value smaller than a reference frequency, and that it is not covered, if the value is larger. A control circuit is provided, which regulates a phase difference between the electrical, transmitted signal and the electrical, received signal to a determined, constant value, at which the oscillatory structure executes oscillations with a resonance frequency.
The control circuit is e.g. formed such that the received signal is amplified and fed back to the transmitted signal through a phase shifter.
Such devices are used for a multiplicity of different applications and are, therefore, exposed to quite varied requirements.